Glutamate decarboxylase (GAD), a specific marker of GABAergic neurones was found to be widely distributed in the septum of the rat. Immunoreactive fibres and terminals were found in all parts of the lateral septal nucleus. GAD positive neuronal cell bodies were most numerous in the medial septal nucleus and the nucleus of the diagonal band. A dense network of met-enkephalin immunoreactive fibres and terminals was seen in the intermediate part of the lateral septal nucleus. In colchicine-injected animals, numerous met-enkephalin-immunoreactive cell bodies were found in the dorsal, intermediate and ventral parts of the lateral septal nucleus. No cell bodies exhibiting Beta-endorphin-like immunoreactivity were found in the septal complex, but the basal parts of both lateral and medial septum contained Beta-endorphin immunoreactive varicose fibres. The results indicate that a large group of GABAergic cells is located in medial septal regions and the terminal fields of septal GABAergic systems are mainly in the lateral septum. The enkephalin-containing neuronal system in the septum most likely consists at least partly of septal interneurones whereas the Beta-endorphin system most probably comes from septal afferents.